A growing number of sophisticated portable telephone units, portable intelligent terminals and the like communication apparatus are being developed. These portable apparatus are requested to be compact in size, light in weight and slim in the overall profile. A wiring board used in such apparatus has been mounted with semiconductor chips, resistors, capacitors, sensors and the like electronic components on its both surfaces. In addition to the basic function expected for the wiring boards, some of the advanced circuit boards contain such circuit components within the inside of the board for better serve the concurrent trends in the apparatus pursuing more compactness and slimness.
Japanese Patent Unexamined Publication No. 2002-261421, for example, discloses a method of incorporating an electronic component within the substrate of a circuit board. FIG. 6A through FIG. 6D are cross sectional views used to describe the manufacturing method. First, as FIG. 6A shows, semiconductor chip 21 and electronic component 23 are placed at their specified locations on the main surface of resin substrate 24, which is made of a thermoplastic resin material. Semiconductor chip 21 is of course among the electronic components, but the semiconductor chip and the electronic component are treated as separate items in the present description. In this conventional example, semiconductor chip 21 is disposed on resin substrate 24 so that the bump electrodes 22 of semiconductor chip 21 face to the main surface of resin substrate.
Then, as FIG. 6B shows, resin substrate 24 mounted with semiconductor chip 21 and electronic component 23 is loaded on a heat-pressing equipment equipped with a pair of heat press plates 25 and 26. Resin substrate 24 is heated by lower heat press plate 26, to have it softened. When upper heat press plate 25 is lowered, semiconductor chip 21 and electronic component 23 are injected into resin substrate 24. The pair of heat press plates 25, 26 is cooled to have resin substrate 24 cured. Thus resin substrate 24 as illustrated in FIG. 6C is provided; wherein, semiconductor chip 21 and electronic component 23 are injected into and embedded.
Next, as FIG. 6D shows, electrical wiring patterns 27 are formed to include semiconductor chip 21's bump electrode 22 and electronic component 23's electrode terminal 231 by, for example, applying a conductor paste using a screen-printing process and hardening it. This completes a finished circuit board; where, semiconductor chip 21 and electronic component 23 are injected into resin substrate 24 and connected by electrical wiring patterns 27 of a certain specific pattern.
As to a material for resin substrate 24, polyethylene terephthalate (PET) or the like polyester material, polyolefin, vinyl chloride, polycarbonate, acrylonitril butadiene styrene, etc. may be used. In a case where polyethylene terephthalate (PET) resin is used for the substrate, semiconductor chip 21 and electronic component 23 can be injected into resin substrate 24 at an approximate temperature 160° C., pressure 400N.
Japanese Patent Unexamined Publication No. 2001-57408 teaches a method which uses a provisional substrate in injecting a semiconductor chip in resin substrate. FIG. 7A through FIG. 7E are cross sectional views used to describe the important process steps of the manufacturing method.
In the first place, as FIG. 7A shows, electrical wiring patterns 27 are formed on the main surface of provisional substrate 28 by, for example, screen-printing a conductor paste and hardening it. Then, as FIG. 7B shows, the bump electrodes 22 of semiconductor chip 21 are connected to electrical wiring patterns 27 by means of a generally practiced connecting method.
As FIG. 7C shows, resin substrate 29 of a thermoplastic resin is pressed onto provisional substrate 28 so that semiconductor chip 21 is involved into the resin substrate. In practice, provisional substrate 28 mounted with semiconductor chip 21 and resin substrate 29 are overlaid together and placed between a pair of heat press plates for a press-heating treatment.
At this stage, as FIG. 7D shows, heat sink 30 is provided at the reverse surface of resin substrate 29. Heat sink 30 may be disposed to be sticking on the surface of resin substrate 29 by heating at least either one of resin substrate 29 and heat sink 30, while keeping provisional substrate 28 as it is. And then, provisional substrate 28 is removed to complete a finished circuit board as shown in FIG. 7E; where, both semiconductor chip 21 and electrical wiring patterns 27 have been injected into resin substrate 29. Since the entire items including electrical wiring patterns 27 are injected and embedded within resin substrate 29, the finished circuit board thus manufactured is thinner as compared to those provided through other methods. Furthermore, the above-configured circuit board can be stacked easily for pluralities into a multi-layered circuit board.
Other method of injecting has also been developed; namely, a substrate is provided using a half-cured thermosetting resin material, and a semiconductor chip is injected into it by raising the temperature of the substrate to be higher than the softening point. This method is suitable to the manufacture of such circuit boards which contain only a small number of components; for example, an IC card circuit board. However, for the manufacture of other types of circuit boards into which many electronic components are injected at different steps during the production, it seems difficult to use the method since it is difficult to provide appropriate processing conditions for each of the different process steps.
These conventional methods use a thermoplastic resin or a thermosetting resin for the material of the resin substrate. When a thermoplastic resin is used, it is easy to inject a semiconductor chip and an electronic component into the substrate. So, the methods are suitable to the manufacture of such cards as IC cards, memory cards, etc., where the cards are not supposed to undergo a high temperature treatment anymore after the injection of semiconductor chip is finished.
However, a thin and compact circuit board which contains many semiconductor devices and electronic components is requested to be free from thermal deformation due to thermal hysteresis during the manufacture and to insure a high level dimensional accuracy. In order to satisfy the above requirements, a thermoplastic resin of high softening point has to be selected. But, the use of such a material necessitates a high temperature and a strong pressure in injecting semiconductor chips or electronic components, which would inevitably endanger the product reliability.